Multiple-stage, rotary-shaft ferrofluid seals, such as those described in U.S. Pat. No. 3,620,584, issued Nov. 16, 1971, are often employed to provide a ferrofluid seal about a rotary shaft between a high- and low-pressure environment or different fluid environments, such as, for example, to provide a seal about a rotary shaft which extends from the atmosphere, as the high-pressure environment, into a vacuum chamber as the low-pressure environment. The basic components of a ferrofluid magnetic seal are a permanent magnet, two magnetically permeable pole pieces, a magnetically permeable shaft element, typically a rotary-shaft element which extends between the two environments to be sealed, and a ferrofluid contained to form a ferrofluid O-ring at the end of the pole pieces. In a multiple-stage ferrofluid seal, the shaft or the pole pieces consist of a plurality of teeth or ridges, so that a plurality of separate ferrofluid stages as seals are formed at the radial gap between the surface of the rotating shaft and the one end of the pole piece. The pole piece, the shaft and the permanent magnet provide for concentrated magnetic flux in the radial gap under each stage of the multiple-stage ferrofluid seal. In an ideal situation, all of the magnetic flux lines are confined under each particular stage, and none extend into the interstage volume between the individual stages. Typically, the field strength under a stage generally is 8.times.10.sup.5 oersteds per centimeter or less, such as 8.times.10.sup.5 to 3.times.10.sup.4. The ferrofluid employed is magnetically entrapped under each of the stages, and a series of liquid sealing O-rings is formed, with intervening regions or interstage volumes filled with air. Each of the seal stages can sustain a pressure differential, such as typically a pressure differential of about 0.2 atmospheres. The individual stages act in a cumulative series, to provide a total pressure capacity for the particular multiple-stage ferrofluid seal.
Where a ferrofluid multiple-stage seal is employed in a vacuum application; that is, between a vacuum environment as a low pressure and an atmosphere or a higher than atmosphere as the high pressure, the multiple-stage seal is normally designed to sustain a pressure differential greater than that between the high and low atmosphere, and typically, when the high pressure is the atmosphere and the low pressure is the vacuum, to sustain a pressure differential of greater than about 2 atmospheres, to permit a safety margin in the operation of the seal. After the initial pumping down of the vacuum system to the desired vacuum level, the seal should operate such that there should be no release of air into the pumped-down vacuum system from any interstage region. However, prior-art, conventional ferrofluid seals employed in high-vacuum systems, typically those systems of 10.sup.-6 TORR or less, either under static or dynamic conditions, may permit a burst of air through the seal and to be introduced into the vacuum system. The burst of air occurs at various periodicities, depending on the seal design, ferrofluid inventory and operational conditions. Furthermore, when the ferrofluid seal is started or is employed for the first time, after being in a static condition, a burst of air is also introduced into the vacuum system. In modern, high-vacuum processing systems, these occurrences of burst of air into the system present limitations on the employment of ferrofluid multiple-stage seals.
Therefore, it is desirable to provide for a nonbursting, multiple-stage ferrofluid seal which may be employed in systems to separate high and low pressures, and typically employed to provide an effective nonbursting seal between a vacuum chamber and the outside atmosphere.